The IRS limits the amount of money you can contribute to retirement plans each year, and these limits can change annually. Here are the 2018 limits for the Thrift Savings Plan (TSP).
TSP Contribution Limits for 2018:
Elective Deferral Limit: $18,500 Applies to combined total of traditional and Roth contributions.
Annual Addition Limit $55,000 An additional limit for tax-exempt pay earned in a combat zone.
Catch-up Contribution Limit $6,000 The maximum catch-up contributions that can be contributed in a given year by participants age 50 and older.
Roth contributions are subject to the elective deferral limit ($18,500 for 2018) even if they are contributed from tax-exempt pay. If you want to contribute tax-exempt pay toward the annual additions limit, you will have to elect traditional contributions for any amount over the elective deferral limit.